Medium voltage converters are typically employed to transform AC power of fixed frequency into AC power of varying frequency, or vice versa. The AC power of fixed frequency is provided by an electric grid, while the AC power of varying frequency is used to supply loads, such as electrical AC machines such as asynchronous machines, synchronous machines or doubly-fed machines.
In most configurations, the frequency transformation is carried out in a two-step approach: First, the AC power of fixed frequency is rectified to DC power and subsequently the DC power is inverted into AC power of the desired frequency. In the power generation mode, the power flow is reversed and the varying-frequency AC power of the load is rectified to DC power and subsequently inverted into fixed-frequency AC power of the grid.
One type of medium voltage converter is a load commutated converter, also referred to as line commutated inverter, solid-state frequency converter or static frequency converter. While being a mature technology, load commutated converters are a good choice in high power applications, due to their high efficiency, simplicity, proven reliability and wide speed and power range.
Load commutated converters are employed by various industries such as the mining, the metals or the oil and gas industries. Load commutated converters are often employed at remote places, where the grid conditions may be far from ideal. Long cables to the power generation may result in a weak grid, i.e. the grid voltage may have a relatively high dependence on the grid current. Weather conditions, line interruptions and the consumption pattern of other large power consumers in the vicinity of a load commutated converter may result in brownouts, in the following also referred to as undervoltage condition, grid voltage sags, voltage dips or temporary power losses.
Processes which are powered by a load commutated converter may be quite sensitive to a loss of drive torque. For example, load commutated converters are used to drive assets in oil and gas industry such as gas pipeline compressors. A voltage dip may lead to tripping all compressors of a plant which is especially bad as the process needs to be stopped completely and then restarted.
There exists a number of approaches to tackle grid undervoltage conditions. U.S. Pat. No. 4,475,150 describes as protection measure against grid undervoltage, where the firing of the line side converter is inhibited during undervoltage conditions. In U.S. Pat. No. 4,642,546, the normal firing is inhibited until the DC link current has decayed. U.S. Pat. No. 4,272,816 describes a hardware implementation of procedure for interrupting the power line in case of a detected overcurrent. In U.S. Pat. No. 4,427,934, a torque reference limiter (which consequently limits the current reference) is described, which becomes active for high stator flux magnitudes. In U.S. Pat. No. 4,237,531, a protection system against overvoltage at the machine-side converter semiconductors is described, which inhibits the firing of the machine-side converter and adapts the firing of the line-side converter.
U.S. Pat. No. 4,420,719 discloses a method for controlling a load commutated converter, which interconnects an AC power grid with a motor. More concrete, U.S. Pat. No. 4,420,719 uses a threshold in the DC current to change the behaviour of the system.
GB 2 034 940 A shows a control of induction heating and melting furnaces using load commutated converters and the operating methods thereto.